Problem: Christopher is 18 years older than Kevin. For the last four years, Christopher and Kevin have been going to the same school. Fourteen years ago, Christopher was 3 times older than Kevin. How old is Christopher now?
Explanation: We can use the given information to write down two equations that describe the ages of Christopher and Kevin. Let Christopher's current age be $c$ and Kevin's current age be $k$ The information in the first sentence can be expressed in the following equation: $c = k + 18$ Fourteen years ago, Christopher was $c - 14$ years old, and Kevin was $k - 14$ years old. The information in the second sentence can be expressed in the following equation: $c - 14 = 3(k - 14)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $c$ , it might be easiest to solve our first equation for $k$ and substitute it into our second equation. Solving our first equation for $k$ , we get: $k = c - 18$ . Substituting this into our second equation, we get the equation: $c - 14 = 3($ $(c - 18)$ $ -$ $ 14)$ which combines the information about $c$ from both of our original equations. Simplifying the right side of this equation, we get: $c - 14 = 3c - 96$ Solving for $c$ , we get: $2 c = 82$ $c = 41$.